GEOCHEMISTRY OF METAMORPHOSED PROTEROZOIC(?) MAFIC
DIKES AND SILLS, HIGHLAND MOUNTAINS, MONTANA
STEPHANIE MOORE, Smith College
Proterozoic(?) intrusions of basaltic magma into quartzofeldspathic gneiss now outcrop in
the Highland Mountains of southwestern Montana in the northwestern quadrant of the Wyoming
Province. These dikes, which O’Neill (1988) has called metabasites, crosscut gneissic banding
with sharp contacts, indicating that the metamorphism that caused the gneissic banding predates
the basaltic intrusions into cold rocks. Some dikes contain their own foliation and almost all
dikes are garnet bearing, demonstrating that the dikes were metamorphosed in a second
metamorphic event. Several of these fine-grained, garnet-bearing, rusty-weathering,
metamorphosed mafic dikes and sills (MMDS) clearly have been folded, but many are exposed
only as isolated low-outcrops.
Chemically, the Highland MMDS are subalkaline tholeiites. Fractional crystallization is
evidenced by a Mg/(Mg+Fe) molar ratio range from 0.37 to 0.53 and a P2O5 range of 0.06 to
0.15 wt%. The ratio of P2O5 to Zr is virtually constant, making them similarly incompatible with
the fractionating minerals and indicating a common origin for the MMDS. If that is so, since
both P2O5 and Zr vary by at least a factor of two, at least fifty percent of the liquid must have
been crystallized between the first and last intrusion. Variation of major element chemistry is
consistent with significant fractionation of orthopyroxene and plagioclase with lesser amounts of
clinopyroxene being removed. Because SiO2 decreases as Mg/(Mg+Fe) decreases, olivine is not
a significant fractionating mineral.
The MMDS of the Highland Mountains are similar in field occurrence and bulk
chemistry to the MMDS of the nearby Tobacco Root Mountains. Plagioclase, orthopyroxene and
clinopyroxene were also fractionated from the Tobacco Root Mountains MMDS, which intruded
2.4 Ga gneisses at 2.06 Ga during continental rifting of the Wyoming Province, and were
metamorphosed at 1.77 Ga during the closing of an ocean basin in the Big Sky Orogeny (Brady
et al., 2004). Because of their proximity and strong similarities, the two suites of MMDS may
have a common origin. They differ, however, in the smaller ranges of TiO2, Ni, Zr, and P2O5 in
the Highland Mountains MMDS compared to the ranges in the Tobacco Root Mountains
MMDS.